A number of prominent combat veteran organizations slammed the Republican party’s presidential nominee Donald Trump on Monday after he criticized the Muslim-American parents of an Army captain killed in Iraq while saving his fellow soldiers.

The Veterans of Foreign Wars (VFW), the nation’s oldest and largest veterans organization, said Trump was out of bounds for insulting Gold Star parents.

“Election year or not, the VFW will not tolerate anyone berating a Gold Star family member for exercising his or her right of speech or expression,” VFW leader Brian Duffy said in a statement.

Vet Voice Foundation — a non-profit that promotes political activity among military veterans — told The National Memo on Monday that Trump’s behavior was “stunning.”

Meanwhile, Vets VS Hate — a grassroots network of veterans against Trump — said the GOP nominee’s comments were “unacceptable.”

“The idea that the president of the United States would show disrespect or slander or attack our parents, no matter what they have to say, is fundamentally unacceptable,” group organizer Alexander McCoy said.

And a group of Gold Star families demanded an apology from Trump in an open letter published published overnight. The letter — signed by family members of 11 service members who were killed in Iraq — slammed the GOP nominee calling his recent comments on the Khans “repugnant.”

In the 13 months since he descended an escalator to announce his bid for the White House, Donald Trump has managed to offend Mexicans, Muslims, Jews, veterans, African-Americans, women and the disabled. To the surprise of pundits and political strategists, his poll numbers continued to rise.

But just 100 days before the general election, Trump may have finally crossed a line in a way he can’t walk back.

Trump, critics say, went too far when he attacked Khizr and Ghazala Khan, whose son Humayun died in Iraq in 2004 after saving hundreds of his fellow soldiers.

Khizr Khan — who criticized Trump for his proposed ban on Muslims entering the U.S. during the Democratic National Convention in Philadelphia last week — is now appealing to Republican voters, urging them to disavow their nominee.

“This candidate is void of empathy, so I will continue to reach out to Republican voters to refrain from voting for this candidate,” Khan told NBC News on Monday. “He needs to mend his way.”

And Arizona Sen. John McCain, a former prisoner of war who was himself the target of a Trump attack last year, also weighed in on Monday, saying in a statement that Trump did not have “unfettered license to defame those who are the best among us.”

Donald Trump vs The Khans: A Timeline (Photos)

Here's a timeline of Donald Trump's fight with Muslim-American immigrants Khizr and Ghazala Khan, who have criticized his proposal to temporarily bar Muslims from entering the United States.

Getty Images/NBC

July 28--Convention speech

Khizr Khan appears at the Democratic National Convention with his wife Ghazala. The Pakistani immigrants criticized Donald Trump's anti-Muslim rhetoric and invoked the name of their son Humayun Khan -- who sacrificed himself in Iraq in 2004 to save his unit. Khizr Khan said Trump has "sacrificed nothing" and offered Trump his personal copy of the U.S. Constitution.

In an interview with George Stephanopoulos, Trump implied that Ghazala Khan did not speak at the convention because her husband would not allow it. He also said that he had made sacrifices, namely building his business empire and creating "thousands of jobs."

Ghazala Khan published an op-ed in the Washington Post saying she declined to speak at the convention because she was overwhelmed with emotion at the memory of her son's death. "Donald Trump said he has made a lot of sacrifices. He doesn’t know what the word sacrifice means," she wrote.

Trump's running mate, Indiana Gov. Mike Pence, issued a statement in Trump's defense that read in part, "Donald Trump and I believe that Captain Humayun Khan is an American hero and his family, like all Gold Star families, should be cherished by every American."

Khizr Khan made the interview rounds, criticizing Trump for his statements against his family and Muslims in general. Trump responded with more tweets, one of which read, "Mr. Khan, who does not know me, viciously attacked me from the stage of the DNC and is now all over T.V. doing the same - Nice!"

Arizona Senator and Vietnam veteran John McCain slammed Trump and defended the Khans in a statement. "While our party has bestowed upon him the nomination, it is not accompanied by unfettered license to defame those who are the best among us," he said.